1. Field of the Invention
The present invention relates to a receiver of a radio communication system and, more particularly, to a diversity receiving apparatus and method.
2. Description of the Related Art
A diversity technique is used to improve the quality of a received signal in a digital radio communication system. In general, a diversity system includes two or more antennas to select or switch a good quality signal or to combine received signals, so that a better quality signal is obtained.
Diversity receiving methods include a switching method, an in-phase combining or equal-gain combining method, and a maximal ratio combining method.
Selection diversity refers to selectively receiving the most powerful signal among signals received by a plurality of antennas. Thus, if some signals received by the plurality of antennas are degraded below a reception sensitivity, the best signals are selected so that a reception signal quality above a certain level can be constantly obtained.
In order to use selection diversity, a controller of a receiving party should recognize the strength of a signal received by the antenna in real time. If this is difficult, a switching diversity method can be used instead. That is, if strength of a reception signal is degraded below a threshold value while the signal is being received by a first antenna, then reception is switched to a second or a third antenna. Switching diversity ensures the improvement of the quality of a reception signal in proportion to that of the selection diversity.
Though selection diversity and switching diversity (referred to as “level comparing diversity”, hereinafter) are expected to improve signal quality in a fading environment where strength of the reception signal changes in time, in a static environment where strength of the reception signal does not change, the above method do not provide for much improvement.
The in-phase combining diversity is a method in which a phase of a signal received by each antenna is controlled and combined, and the combined signal is used as a final reception signal. Since the method uses combining of signals, rather than simply selecting a signal, a remarkable improvement of a signal quality can be obtained compared to the level comparing diversity. The in-phase combining technique requires a phase shifter for adjusting a phase of a signal and obtains an improvement in a reception signal quality even in the static environment as well as in the fading environment.
FIG. 1 illustrates the construction of a diversity receiving apparatus (Korean Patent Laid Open Publication No. 1996-0016194 entitled ‘diversity receiving apparatus using phase compensation’), which adopts an in-phase combining diversity method.
As shown in FIG. 1, the conventional diversity receiver includes: a phase shifter 10 for shifting the phase of a signal (first signal) received through a first antenna 6 to one of 0°, 90°, 180° and 270°; a signal combiner 20 for combining the phase-shifted first signal and a signal (second signal) received through a second antenna 8; a tuner 30 for selecting a signal from output signals of the signal combiner 20 and improving the signal-to-noise ratio; an amplifier 40 for amplifying the output signal of the tuner 40; a mixer 50 for mixing a signal outputted from the amplifier 40 and a signal of a local oscillator and outputting an intermediate frequency (IF) signal; an intermediate frequency amplifier 60 for amplifying a signal with a certain frequency band from the intermediate frequency signal; a received signal strength indicator(RSSI) 70 for detecting a strength of the signal outputted from the intermediate frequency amplifier 60; and a controller for comparing the detected strength of the signal and a reference value and controlling the phase shifter 10.
The signal (first signal) received through the first antenna 6 is phase-shifted by 0°, 90°, 180° and 270° while passing through the phase shifter 10 and then transmitted to the signal combiner 20. Then, the signal combiner 20 in-phase combines the phase-shifted first signal and the signal (second signal) received through the second antenna 8. The combined signal is transmitted to the mixer 50 through the tuner 30 and the amplifier 40, and converted into an intermediate frequency signal in the mixer 50. The intermediate frequency signal of the mixer 50 is transmitted to a demodulator after passing through the intermediate frequency amplifier 60.
The received signal strength indicator 70 detects the strength of the signal outputted from the intermediate frequency amplifier 60, and the controller 80 compares the detected strength of the signal with a reference value and shifts a phase of the phase shifter 10 so that the detected strength of the signal can become the greatest.
When a phase difference between the first signal and the second signal is not exactly 0°, 90°, 180° or 270°, combination of the two signals (first and second signals) causes a combination loss. In addition, due to a time delay, such as a multi-pass fading or a phase delay, the combined signal may have rather less strength. For example, in a case that a phase difference between the first and the second signals is 180°, combination of first and second signals may result in cancellation of the combined output signal.
Using the in-phase combing (or equal gain combing) method, the conventional diversity receiver may have a problem with much power difference between the received two signals. If one of two combined signals (S1 and S2) has a very small power and the two signals have a similar amount of noise components, if the two signals (S1 and S2) are combined, the combined signal obtains some gain but the noise component of the combined signal is doubled, resulting in degradation of the signal-to-noise ratio of the combined signal. For this case, a level comparing diversity method is favored in which a signal with high power from two reception signals is selected.
FIG. 2 is a graph of a signal affected by a Rayleigh fading. As shown in FIG. 2, the Rayleigh fading occurs in an actual radio environment, and since the radio environment changes rapidly, a signal should be compensated quickly. However, in the conventional art, the position where a signal becomes the strongest is searched by shifting the phase of signal, for which four times of phase shifting at the maximum is performed with 0°, 90°, 180° and 270° to measure the strength signal. With much time taken for compensating the phase, the conventional art encounters many problems in the actual radio environment. Thus, methods and systems are needed to overcome the mentioned problems.